The invention relates to a vertebral anchor device comprising a bone anchor screw having a screw body and a screw head mounted to move on the screw body.
The bone anchor device of the invention is designed particularly but not exclusively for treating curvatures or deviations of the spine, such as scolioses.
Scoliosis is a deviation of the spine whereby the vertebrae define a curvature that is classically S-shaped. It is the consequence of rotation of vertebrae. More specifically, the spine, i.e. the vertebral column, has an inclination in the frontal plane or “coronal plane”, it also being possible for the vertebrae affected by the deviation to have a rotation in the horizontal plane or “transverse plane.”
To reduce the deviation of the spine, in particular in severe scolioses, the surgical technique of spinal arthrodesis or “spinal fusion” is used. The aim of spinal arthrodesis is to stop the progression of the curvature of the spine and to reduce the spinal deformation by restoring the stability and alignment of the spine.
The spinal deformation is reduced by using bone anchor screws in the vertebrae concerned, and link rods mounted on the bone anchor screws. The bone anchor screws are implanted substantially perpendicularly relative to the vertebrae to which they are fastened, while the link rod should be placed substantially parallel to the spine. A clamping nut is then placed on each head to immobilize the link rod to an extent while nevertheless allowing it to move in rotation and in translation on the head in such a manner as to allow the reduction manipulations to take place before the nut is finally tightened, thereby fully immobilizing the link rod on the head.
Straightening (derotation) of the vertebra is performed either directly during final tightening of the nut when the anchor screws used are fixed-head screws or uniplanar screws, or else prior to final tightening when the anchor screws used are polyaxial screws. When polyaxial screws are used, the straightening is achieved by gradually applying tractions or compressions to each of the bone anchor screws implanted on the vertebrae to be straightened by means of appropriate instruments or by moving the link rods in rotation.
Unfortunately, regardless of whether they are fixed-head screws, uniplanar screws, or polyaxial screws, the anchor screws that are used suffer from drawbacks for implementing spine deformation reduction.
The main drawback encountered with fixed-head anchor screws lies in putting the link rods in place. Depending on the positions of the vertebrae, the respective heads of the various bone anchor screws may have different inclinations and directions due to the various orientations of the vertebrae, so that it is difficult to place a link rod through all of the heads of the anchor screws. It is then necessary to make curvature adaptations to the link rods in order to put them in place in the screw heads, and such an operation is difficult in most situations in which the deformation of the spine is considerable.
Polyaxial-head anchor screws make it possible to overcome the drawbacks of fixed-head anchor screws, since the polyaxial heads can be oriented in all directions in three-dimensional space in order to receive the link rods. However, polyaxial-head anchor screws suffer from the drawback of not making it possible to straighten the vertebrae directly, since the vertebrae remain in place during the final tightening of the nut. It is thus necessary, once the bone anchor elements, the link rod, and the clamping nuts are in place, to reduce the deviation of the vertebrae by techniques of derotating the vertebrae and of bending the link rod. The operation of straightening vertebrae with polyaxial screws thus constitutes a whole separate operation unlike the operation performed with fixed-head screws that is performed jointly with the operation of tightening the nuts.
Uniplanar screws are characterized by moving in a single plane. Although, like fixed-head screws, uniplanar screws allow the vertebrae to be straightened during the operation of tightening the nuts, they only offer limited movement between the screw head and the screw body (movement in a single plane) so that, when the vertebral length to be corrected is large and when it is necessary to use a plurality of anchor screws, the movement in the plane is generally insufficient to enable the link rods to be put in place. It is then necessary, like it is with fixed-head screws, to make curvature adaptations to the link rods.